New Fossils Reveal the First Of Man's Walking Ancestors

By JOHN NOBLE WILFORD

Published: August 17, 1995

Fossil discoveries in Kenya have revealed a new species of human ancestors that lived four million years ago, were hardly any bigger than chimpanzees, bore striking resemblances to both apes and evolving man and, perhaps most significantly, were already standing erect and walking on two legs much like modern humans.

Leg and arm bones found near Lake Turkana, paleontologists said, provide the earliest direct and unambiguous evidence for upright walking, or bipedalism, by any members of the human family tree. Until now, the oldest evidence has been the 3.6-million-year-old footprints at Laetoli in Tanzania, together with tiny foot bones from South Africa first reported last month that are about the same age.

By placing the emergence of bipedalism further back by about half a million years, the discovery supported the growing impression that this novel mode of locomotion might be the defining adaptation that first set human ancestors and their close relatives, known collectively as hominids, apart from the quadrupelal apes. From genetic studies comparing modern apes and modern humans, molecular biologists have established that the split between the two lineages occurred five million to seven million years ago.

The discovery also crowds and probably complicates the base of the hominid family tree. It could provoke a new round of controversy among scientists trying to reconstruct human genealogy.

For two decades, the earliest known hominid was a species represented most famously by the Lucy skeleton. Known as Australopithecus afarensis, this appeared to be the sole hominid species from 3.9 million to 2.9 million years ago. Reading the DNA clock in the genes based on guesses about the rate of mutation, molecular biologists started asserting that the split from the apes was far more recent than most paleontologists believed -- a mere 5 to 7 million years ago. Thus inspired, paleontologists searched more diligently for signs of earlier hominid life by looking in Africa for fossil-bearing sediments of this critical age.

With today's announced discovery, two new species have become known in the last year. A 4.4-million-year-old species from Ethiopia was identified and described last year; first lumped in the Australopithecus genus, it was recently reassigned to a new genus and given the name Ardipithecus ramidus. These hominids may have walked upright, but no direct evidence for this has yet been reported.

Now, in a report being published today in the journal Nature, a team led by Dr. Meave Leakey of the National Museums of Kenya announced the discovery of the second new hominid, which has been named Australopithecus anamensis. The fossils of 21 specimens, including teeth and parts of the jaw, were found at two sites near Lake Turkana, Kanapoi and Allia Bay. The name "anam" means lake in the Turkana language.

If the name of the principal discoverer has a familiar ring, it may be because she is the wife of Richard Leakey, the prominent Kenyan fossil hunter who has left the field because of a crippling airplane accident and new interests in wildlife preservation and politics. He is the son of Mary Leakey and the late Louis S. B. Leakey, whose many discoveries this century established East Africa as the ancestral grounds for many human forerunners. The British-born Meave Epps Leakey, following in the family tradition, is an experienced paleontologist with a doctorate in zoology and years of fossil-hunting behind her.

The co-authors of the Nature report are Dr. Alan Walker, an anatomist at Pennsylvania State University who has worked for years with the Leakeys, and two geologists, Dr. Craig S. Feibel of Rutgers University in New Brunswick, N.J., and Dr. Ian McDougall of the Australian National University in Canberra. Their research was supported by the National Science Foundation and the National Geographic Society.

Geologists have dated the new specimens at 3.9 million to 4.2 million years old, making them intermediate in time between the ramidus species discovered by Dr. Tim White of the University of California at Berkeley and the later afarensis species, whose Lucy fossils were found in 1974 by Dr. Donald C. Johanson, a paleontologist now at the Institute for Human Origins in Berkeley.

The new species seemed to be a blend of primitive and advanced characteristics. The jaw, with its shallow palate, large canines and the placement of teeth, harks back to the apes. So do the extremely small ear openings in the skull. But the tooth enamel is much thicker than that of the apes or the ramidus hominid, thus resembling a feature of later hominids in the Homo lineage. And the tibia, or shinbone, and humerus, the upper-arm bone, are especially advanced. In one of the most telling clues to bipedality, the upper end of the shinbone is shaped to bear more weight than a four-legged animal would require.

Dr. Ian Tattersall, a paleontologist at the American Museum of Natural History in New York City, said the new fossils seemed to be "more plausibly hominid than ramidus is" and should advance understanding of when and where upright posture first evolved.

Dr. Johanson, often a rival of the Leakeys in finding and interpreting early human fossils, endorsed the conclusions drawn from the new finds.